Yellow-Orange-Emitting CaAlSiN3:Ce3+ Phosphor: Structure, Photoluminescence, and Application in White LEDs

The crystal structure and photoluminescence properties of undoped and Ce3+-doped CaAlSiN3 as well as the application of white-light LEDs are reported. CaAlSiN3 and CaAlSiN3:Ce3+ have been synthesized, starting from Ca3N2, AlN, Si3N4, and CeN or CeO2, with and without Li3N, by a solid state reaction at 1700 degrees C for 4 h under high purity nitrogen atmosphere. Instead of an ideal CaAlSiN3, a more appropriate formula is proposed to be CaAl1-4 delta/3Si1+delta N3 (delta approximate to 0.3-0.4) with an Al/Si ratio of about 1:2 on the basis of the bond valence sum calculations, in which Al/Si is disorderly occupied on the 8b site within Cmc2(1) space group. Ce3+ can be incorporated into the host lattice of CaAlSiN3, and the estimated maximum solubility of Ce3+ is about x = 0.02 (e.g., 2.0 mol % with respect to Ca) of Ca1-2xCexLixAlSiN3. CaAlSiN3: Ce3+ can be efficiently excited by blue light (450-480 nm) and yields yellow-orange emission with a broadband peaking in the range of 570-603 nin, originating from the 5d' - 4f' transition of Ce. With an increase of Ce concentration, the emission band of Ce shifts to longer wavelengths due to the increased Stokes shift corresponding to structural relaxation and energy transfer of Ce3+. Upon excitation in blue light range (450-480 nm), the absorption and external quantum efficiency are about 70% and 56%, respectively, for both Ca1-xCexLixAlSiN3 and Ca1-xCexAlSiN3-2x/3O3x/2 at x = 0.01. In addition, Ca1-2xCexLixAlSiN3 and Ca1-xCexAlSiN3-2x/3O3x/2 show high thermal stability in air with the quenching temperature above 300 degrees C for x = 0.01. Using single CaAlSiN3:Ce3+ as the wavelenuth conversion phosphor combined with a blue InGaN LED-chip (450 nm), warm white-light LEDs can be generated, yielding the luminous efficacy of about 50 lm/W at color temperature 3722 K and the color rendering index (Ra) of 70, which demonstrates that CaAlSiN3:Ce3+ is a highly promising yellow-orange phosphor for use in white-light LEDs.